1. Field of the Invention
The present invention relates to an illumination device and a liquid crystal display device, and more particularly, to the configurations of an illumination device that can uniformly and brightly illuminate a large area with only one light source, and of a liquid crystal display device using the illumination device.
2. Description of the Related Art
Front lights of reflective liquid crystal display devices have been formed in a unit including a light source, an intermediate light guide, a light guide plate, and a case member having a reflective inner surface to hold these members together.
FIG. 15A is a perspective structural view of a conventional liquid crystal display device, FIG. 15B is a plan view of a front light provided in the liquid crystal display device shown in FIG. 15A, as viewed from the viewing side, and FIG. 16 is a cross-sectional structural view of the liquid crystal display device shown in FIG. 15. The liquid crystal display device shown in these figures includes a liquid crystal display unit 120, and a front light 110 disposed on the front side of the liquid crystal display unit 120. Although not shown in detail, the liquid crystal display unit 120 is a reflective liquid crystal display unit that performs display by reflecting light incident from its front side, and holds a liquid crystal layer 123 between an upper substrate 121 and a lower substrate 122 opposing each other. By controlling the alignment state in the liquid crystal layer 123, the light transmitting state is changed to perform display.
The front light 110 includes a flat light guide plate 112, a bar-shaped intermediate light guide 113 disposed at an end face 112a of the light guide plate 112, and a light emitting element 115 disposed at one end face of the intermediate light guide 113. An upper surface of the light guide plate 112 serves as a reflecting surface 112c on which a plurality of prism grooves 114 of wedge-shaped cross section are formed in parallel and in stripes in plan view, and a lower surface thereof serves as an emergent surface 112b from which illumination light for illuminating the liquid crystal display unit 120 is emitted. Each of the prism grooves 114 is composed of a gently inclined face 114a and a sharply inclined face 114b, as shown in FIG. 16.The inclination angle θ1 of the gently inclined faces 114a is set at a fixed value within the range of 5° to 35°, and the inclination angle θ2 of the sharply inclined faces 114b is set at a fixed value larger than the inclination angle θ1 of the gently inclined face 114a. The pitch P of the prism grooves 114 (width of the prism grooves 114) is fixed in the plane of the reflecting surface 112c. The depth “d” of the prism grooves 114 is also fixed in the plane of the reflecting surface 112c. These prism grooves 114 are slightly inclined relative to the end face 112a of the light guide plate 112 in order to prevent moirée fringes.
In the front light 110, light emitted from the light emitting element 115 is applied through the intermediate light guide 113 to the end face 112a of the light guide plate 112, and is introduced into the light guide plate 112. The light is reflected by the inner side of the upper surface 112c of the light guide plate 112 having a prism shape to change the propagating direction thereof, and is emitted from the lower surface 112b of the light guide plate 112 toward the liquid crystal display unit 120.
In portable electronic devices such as personal digital assistants and portable game machines, since the battery driving time has a great influence on the ease of use, liquid crystal display devices used as display sections in the electronic devices have adopted a single-LED front light having only a single light emitting element, as in the front light 110 shown in FIG. 15, in order to reduce the power consumption of the front light. That is, the number of light emitting elements is limited to reduce the power consumption. Furthermore, with size reduction of the portable electronic devices, there has been a request to reduce the thickness of the front light to approximately 1 mm.
It is, however, substantially impossible for such a single-LED front light to uniformly and brightly illuminate a large display region having a diagonal size of several inches or more by a combination of the thin light guide plate and the single light emitting element. That is, in a case in which the light emitting element 115 is provided on one side of the front light 110 shown in FIG. 15, it is first necessary to make incident light uniform in the lengthwise direction of the end face 112a of the light guide plate 112 by the intermediate light guide 113 in order to uniformly guide light from the light emitting element 115 to the light guide plate 112. It is, however, difficult for the intermediate light guide 113 to make light incident on the light guide plate 112 uniform. Moreover, since the amount of light emitted from the light guide plate 112 having the above-described structure is decreased away from the light emitting element 115, it is difficult to obtain uniform light from the entire emergent surface 112b, and to uniformly illuminate the display region of the liquid crystal display unit 120 without causing variations in luminance. This sometimes reduces display visibility.
While there is an increasing demand for a front light using a single light emitting element as a light source in this way, a thin front light that can uniformly and brightly illuminate a large area has not yet been achieved.